// Smartphone Coding

#include"bits/stdc++.h"
#include"atcoder/all"
using namespace std;
using namespace atcoder;
using lint = long long;
using ulint = unsigned long long;
using llint = __int128_t;
#define endl '\n'
int const INF = 1<<30;
lint const INF64 = 1LL<<61;
lint const mod107 = 1e9+7;
using mint107 = modint1000000007;
long const mod = 998244353;
using mint = modint998244353;
lint ceilDiv(lint a, lint b){if(a%b==0){return a/b;}
  if(a>=0){return (a/b)+1;}
  else{return -((-a)/b);}}
lint floorDiv(lint a, lint b){if(a%b==0){return a/b;}
  if(a>=0){return (a/b);}
  else{return -((-a)/b)-1;}}
lint Sqrt(lint x){lint upper = 1e9;lint lower = 0;while(upper - lower > 0){lint mid = (1+upper + lower)/2;if(mid * mid > x){upper = mid-1;}else{lower = mid;}}return upper;}
lint gcd(lint a,lint b){if(a<b)swap(a,b);if(a%b==0)return b;else return gcd(b,a%b);}
lint lcm(lint a,lint b){return (a / gcd(a,b)) * b;}
lint chmin(vector<lint>&v){lint ans = INF64;for(lint i:v){ans = min(ans, i);}return ans;}
lint chmax(vector<lint>&v){lint ans = -INF64;for(lint i:v){ans = max(ans, i);}return ans;}
double dist(double x1, double y1, double x2, double y2){return sqrt(pow(x1-x2, 2) + pow(y1-y2,2));}
string toString(lint n){string ans = "";if(n == 0){ans += "0";}else{while(n > 0){int a = n%10;char b = '0' + a;string c = "";c += b;n /= 10;ans = c + ans;}}return ans;}
string toString(lint n, lint k){string ans = toString(n);string tmp = "";while(ans.length() + tmp.length() < k){tmp += "0";}return tmp + ans;}
vector<lint>prime;void makePrime(lint n){prime.push_back(2);for(lint i=3;i<=n;i+=2){bool chk = true;for(lint j=0;j<prime.size() && prime[j]*prime[j] <= i;j++){if(i % prime[j]==0){chk=false;break;}}if(chk)prime.push_back(i);}}
lint Kai[250001]; bool firstCallnCr = true; 
lint ncrmodp(lint n,lint r,lint p){ if(firstCallnCr){ Kai[0] = 1; for(int i=1;i<=250000;i++){ Kai[i] = Kai[i-1] * i; Kai[i] %= p;} firstCallnCr = false;} if(n<0)return 0;
if(n < r)return 0;if(r<0)return 0;if(n==0)return 1;lint ans = Kai[n];lint tmp = (Kai[r] * Kai[n-r]) % p;for(lint i=1;i<=p-2;i*=2){if(i & p-2){ans *= tmp;ans %= p;}tmp *= tmp;tmp %= p;}return ans;}
#define rep(i, n) for(int i = 0; i < n; i++)
#define repp(i, x, y) for(int i = x; i < y; i++)
#define vec vector
#define pb push_back
#define se second
#define fi first
#define al(x) x.begin(),x.end()
#define ral(x) x.rbegin(),x.rend()
using str=string;


template< typename T >
T extgcd(T a, T b, T &x, T &y) {
  T d = a;
  if(b != 0) {
    d = extgcd(b, a % b, y, x);
    y -= (a / b) * x;
  } else {
    x = 1;
    y = 0;
  }
  return d;
}
struct edge{
    int to;
    lint cost;
};
using graph = vector<vec<edge>>;
vec<lint>dijkstra(int s, graph& g){
	priority_queue<pair<lint,lint>>que;
	vec<lint> cost(g.size(),INF64);
	cost[s]=0;
	que.push({0,s});
	while(!que.empty()){
		auto q=que.top();
		que.pop();
		int v=q.second;
		for(auto e:g[v]){
			if(cost[e.to]>cost[v]+e.cost){
				cost[e.to]=cost[v]+e.cost;
				que.push({-cost[e.to],e.to});
			}
		}
	}
	return cost;
}


int main(){
	int h,w;cin>>h>>w;
	int a[w],b[w];
	mcf_graph<int,int>g(2*w+2);
	int S=0,T=w+w+1;
	rep(i,w){
		cin>>a[i]>>b[i];
		a[i]--;
		b[i]--;
		g.add_edge(S,i+1,1,0);
		rep(j,w){
			int cost;
			if(j>=b[i]-a[i]&&j<=b[i])cost=a[i];
			else if(j<b[i]-a[i]){
				cost=b[i]-j;
			}else{
				cost=j-b[i]+a[i];
			}
			g.add_edge(i+1,w+j+1,1,cost);
		}
		g.add_edge(w+i+1,T,1,0);
	}
	auto f=g.flow(S,T);
	cout<<f.second;
}